Brief introduction to Active filter circuits.

Cuthbert Nyack

Active filters using opamps have some advantages over passive filters particularly at low frequencies.
Active filters can be constructed without large inductors using only opamps, resistors and capacitors.
They can be designed with gain.
For higher order filters the transfer function can be factorized into quadratics and cubics and each factor can be implemented independently. These filters can then be realized by connecting first, second and third order sections in series.

Several circuit configurations are possible using general purpose opamps. Here only a few classic circuits and their application to basic filter transfer functions will be considered.

Some manufacturers produce special opamps designed to simplify filter circuits. These manufacturers provide data sheets on the use of their devices and they will not be considered here.

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